1. Field of the Invention
The present invention relates to a digital imaging apparatus and, more particularly, to a digital imaging apparatus which generates at high speed with high quality an image including pixels smaller than that of an imaging device mounted in the imaging apparatus.
2. Description of the Related Art
Recently, with the rapid proliferation of personal computers, digital cameras as image input devices have grown in demand. In addition, high-image-quality recording apparatuses such as digital video devices as moving image recording devices have been widely used.
There are several factors that determine the image quality of the above electronic still camera. Of these factors, the number of pixels of an imaging element is a significant factor that determines the resolution of a sensed image. For this reason, several electronic still cameras having as many as five million pixels or more have been commercialized. However, five million pixel data is not always required for all applications. On the contrary, images with reduced pixel sizes are often used as images to be displayed on the Web in the Internet.
The transfer times from imaging elements to image memories are a bottleneck in current digital cameras. There are few cameras of models that are large in the number of pixels and can perform high-speed continuous shooting. In addition, since a moving image shooting function is required for a digital camera as an additional function, data needs to be transferred to the memory at high speed. The amount of data to be processed needs therefore to be reduced in advance.
If the number of pixels of an output image is smaller than that of an imaging element, the amount of data to be transferred from the imaging element to the memory can be reduced by limiting the number of pixels to be used in advance or reading out pixels upon averaging them, thereby increasing the memory transfer speed.
According to size reduction by linear interpolation, a large-size image is formed by using all pixels, and a small-size image is formed by linear interpolation.
Such resizing by linear interpolation is preferable in terms of image quality. However, since linear interpolation is performed by capturing the data of all pixels, a large computation amount is required. This technique is not therefore suitable for the above continuous shooting function and moving image shooting function.
As a technique of reducing the amount of data to be read out from a memory, there is available a method of generating a reduced image by attaching an integration function to an imaging element and reading out a small number of data upon averaging them. Jpn. Pat. Appln. KOKAI Publication No. 2001-245141 discloses a high-speed image reduction method using this method.
Jpn. Pat. Appln. KOKAI Publication No. 2001-016441 discloses an apparatus which thins out data when the types of resolutions to be used are limited, and also corrects the distortion of data. This reference also discloses how data corresponding to 400 dpi is generated by an apparatus having a resolution of 600 dpi in an embodiment. If 600-dpi data is directly thinned out, the resultant data is distorted. For this reason, pixel data for correcting positional distortion is generated from the 600-dpi data by linear interpolation.
The method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-245141 is effective in reducing operation at a reduction ratio of about 20% or less. This method, however, cannot remove image distortion due to readout pixel positions by averaging based on integration alone in reducing operation at a high reduction ratio (by about 40% or more). In a wide range of reduction ratios, therefore, it is difficult to form an image with high image quality while making a size change.
The apparatus disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-016441 forms data corresponding to 400 dpi by performing interpolation using all 600-dpi data obtained by scanning. In contrast to this, the present invention is directed to a new technique associated with thinning-out scanning which copes with a case wherein basic-resolution data (600-dpi data obtained by scanning in Jpn. Pat. Appln. KOKAI Publication No. 2001-016441) cannot be completely read out due to a limitation on readout time. For example, a data string with little distortion is generated from a thinned-out data string like that shown in FIG. 6(b) in Jpn. Pat. Appln. KOKAI Publication No. 2001-16441. The present invention therefore differs from the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-016441.